Effect of Polyamines on a Ribonuclease Which Hydrolyzes Ribonucleic Acid at Uridylic Acid Residues

A heat-stable ribonuclease, isolated from Citrobacter species, catalyzes the conversion of polyuridylic acid to cyclic 2',3'-uridylic acid but does not hydrolyze polycytidylic, polyguanylic, or polyadenylic acids. The enzyme, purified some 1500-fold, migrates as a single band after electrophoresis on polyacrylamide gels and exhibits an absorption maximum and minimum at 281 and 248 nm, respectively. Evidence from studies with synthetic polyribonucleotides and with yeast ribonucleic acid suggests that the nuclease has a distinct predilection for internucleotide bonds containing uridylic acid at either end of the phosphodiester linkage. Enzyme activity is affected profoundly by the polyamines, putrescine and spermidine. Both of these substances as well as the ammonium ion stimulate enzyme activity considerably. In addition, spermidine appears to alter the high degree of specificity exhibited by the enzyme to an activity much more random in nature. Enzyme activity is inhibited by polyguanylic acid and by the double- and triple-stranded complexes, polyadenylate + polyuridylate and polyadenylate + 2 polyuridylates, respectively. The inhibition of enzyme activity induced by these polynucleotides was found to be competitive, and activity could be restored completely by spermidine.